1. Field of the Invention
This invention relates to a stabilized aromatic copolyester composition, more specifically, to a stabilized aromatic copolyester composition including a phosphorus-containing compound.
2. Description of the Prior Art
The aromatic copolyester used in this invention is obtained from a mixture of terephthalic acid and/or functional derivatives thereof and isophthalic acid and/or functional derivatives thereof (with the terephthalic acid/isophthalic acid unit mole ratio being about 9:1 to about 1:9) and a bisphenol of the following general formula (I) ##STR2## wherein --X-- is selected from the group consisting of --O--, --S--, --SO.sub.2 --, --SO--, --C0--, an alkylene group containing 1 to 4 carbon atoms, and an alkylidene group containing 1 to 4 carbon atoms, and R.sub.1 , R.sub.2, R.sub.3, R.sub.4, R.sub.1 ', R.sub.2 ', R.sub.3 ' and R.sub.4 ', which may be the same or different, each represents a member selected from the group consisting of a hydrogen atom, a chlorine atom, a bromine atom and an alkyl group containing 1 to 4 carbon atoms, or functional derivatives thereof.
Known methods for producing such an aromatic copolyester include, for example, an interfacial polymerization method which involves mixing an aromatic dicarboxylic acid chloride dissolved in a water-immiscible organic solvent with an alkaline aqueous solution of a bisphenol, a solution polymerization method which comprises heating a bisphenol and an acid chloride in an organic solvent, and a melt polymerization method which comprises heating a phenyl ester of an aromatic dicarboxylic acid and a bisphenol, e.g., as disclosed in U.S. Pat. Nos. 3,884,990 and 3,946,091.
It is also known that aromatic copolyesters derived from aromatic dicarboxylic acids and bisphenols have superior properties to polyesters derived from aromatic dicarboxylic acids and aliphatic alkylene glycols. Specifically, these copolyesters have many superior characteristics, e.g., mechanical properties such as tensile strength, elongation, flexural strength, flexural recovery and impact strength, heat-distortion temperature, dimensional stability, electrical properties, and fire retardancy. Because of these superior properties, these copolyesters are known to be useful in a wide range of fields as various molded articles, films, fibers, and coating materials obtained by extrusion molding, injection molding, etc.
The aromatic copolyesters obtained by the above methods frequently assume yellow or brown colors in their as-prepared state. It is also noted that aromatic copolyesters turn yellow when heated during injection molding, extrusion molding and other shaping operations which are used to form ordinary molded products or films. Such coloration is extremely inconvenient when a lack of color in the resulting molded articles is important. Furthermore, when a pigment is incorporated into a molding composition to obtain a desired color, the color of the final product frequently is quite different from the desired color when the molding composition is colored.
The aromatic copolyesters used in this invention have a high heat distortion temperature, and therefore, are frequently used under high temperature conditions. The above-described coloration, however, proceeds generally under high temperature conditions and impairs the transparency of the molded product. Consequently, such a product cannot be used in applications which require transparency and a lack of color at high temperatures.
Such a coloration is also considered to occur with the decomposition of the polymer. Thus, coloration causes an irregular reduction in the logarithmic viscosity number of the polymers constituting the molded articles and deteriorates the useful properties of these polymers. Coloration is therefore extremely disadvantageous in obtaining products which are uniform. The techniques known to obviate this problem are the addition of sodium dithionite (e.g., as disclosed in Japanese Patent Publication No. 11297/63) of polyphenylene or active anthracene (e.g., as disclosed in Japanese Patent Publication No. 23418/70). These methods, however, do not produce entirely satisfactory results when applied to the aromatic copolyesters having high processing temperatures as used in this invention.
Compounds of the naphthylamine, diphenylamine, ethylenediamine, and aromatic amine types are used as antioxidants for rubbers. When used as antioxidants for plastics, these amine type compounds can reduce the decomposition of plastics, but, unfortunately, they also degrade their color. Hence, the resulting molded articles are heavily colored. Further, organotin mercaptide compounds as disclosed in Japanese Patent Application (OPI) No. 51048/73, various kinds of acid anhydrides as disclosed in Japanese Patent Application (OPI) No. 23254/74, epoxy compounds as disclosed in Japanese Patent Application (OPI) No. 22754/76 or of a phosphite compound (e.g., as disclosed in Japanese Patent Application (OPI) No. 16558/77, corresponding to U.S. patent application Ser. No. 110,050, filed July 30, 1976 and the like are known as a coloration preventing agents for aromatic copolyesters.
Aromatic copolyesters also have the serious defect that when molded products made therefrom are allowed to stand for long periods of time in hot water or steam, water crazing occurs. Water crazing is a phenomenon whereby a clouding or a network of fine cracks appears on the surface of in the interior of molded articles, and this phenomenon has been reported in the literature and elsewhere. Water crazing occurs wholly or partly in molded articles of aromatic copolyesters upon treatment in hot water or steam. The occurrence of water crazing not only causes a loss of the transparency of the molded articles, but also renders them brittle. As a result, the impact strength and break elongation are reduced, and the molded articles easily break under a bending stress or an impact force.
Some methods are known in the art for preventing water crazing in aromatic copolyester molded articles. Among these methods is a method in which polyethylene terephthalate is mixed with the aromatic copolyester and the mixture is melted and formed into molded articles (e.g., as disclosed in U.S. Pat. No. 3,946,091), and a method in which polyethylene-p-hydroxybenzoate is added to the aromatic copolyester (e.g., as disclosed in U.S. Pat. No. 3,884,990).
These methods are effective for preventing the occurrence of water crazing (referred to hereinbelow simply as "crazing") but do not produce an effect of stabilizing polymers to heat and improve their color.